Persistent Sciatic Vein (PSV) remains a rare condition with no associated malformative syndrome or concomitant Persistent Sciatic Artery (PSA). Very few case reports have been published. We report the case of a 73-year-old man who presented to the emergency room with tenderness, edema and pain of the left calf, for up to 3 days. Biology reported an elevated d-dimer level. Doppler ultrasound diagnosed proximal left deep popliteal vein thrombosis, extended to a large vein along the sciatic nerve in the posterior compartment of the thigh, and a patent superficial femoral vein. Anticoagulation therapy was initiated immediately for at least 3 months. PSV remains an understudied condition. While its prevalence is low in the general population, this anatomical condition is associated with some specific clinical situations, such as Klippel-Trenaunay syndrome (KTS) or early varicose recurrence, especially in the posterior thigh area. To our knowledge, this is the first case report of a thrombosed PSV in a patient without a KTS.